Chromium catalyst compositions and polymerization processes therewith

ABSTRACT

Chromium catalyst compositions are provided. These chromium catalyst compositions comprise at least two chromium catalyst systems that have a certain amount of chromium and a support that comprises silica and titania. the supports have certain pore volumes and surface areas. These catalyst compositions are useful for homopolymerizing ethylene or copolymerizing ethylene with a comonomer. The resins produced are useful in film applications.

This application is a division application of application Ser. No.08/134,765, which was filed on Oct. 8, 1993, and which issued as U.S.Pat. No. 5,508,362 on Apr. 16, 1996.

This invention is related to the field of chromium catalystcompositions. This invention is also related to the fields ofpolymerizing ethylene, or copolymerizing ethylene and a comonomer, witha chromium catalyst composition.

The high molecular weight-high density ethylene polymer film market, inthe U.S., is dominated by multicomponent ethylene polymers (colloquiallyreferred to as "bimodal" or "multimodal" polymers) that are produced bymixtures of titanium halide catalysts. These multicomponent ethylenepolymers offer good impact resistance and thus, good down gaugingcapability, along with a high modulus. The advent of thesemulticomponent ethylene polymers has opened up markets previouslycovered by other materials such as paper, thicker gauge ethylenepolymers, and lower density ethylene polymers. However, thesemulticomponent ethylene polymers are produced commercially using cascadereactor systems in which two reactors are linked in series so that a twostage polymerization can take place with different polymerizationconditions in each reactor. This is a more expensive production methodthan a single stage reactor system.

SUMMARY OF THE INVENTION

In order to compete with these multicomponent ethylene polymers it isimportant for a resin to have good impact strength, tear resistance inthe transverse direction, appearance, and processing behavior. To date,chromium catalysts have not been competitive in the high molecularweight-high density ethylene polymer film market. For example, onechromium catalyst may produce a resin that has good film properties, butpoor bubble stability at high production rates and thin gauges. On theother hand, another chromium catalyst may produce a resin that has agood bubble stability at high production rates and thin gauges, but poorfilm properties. However, making a chromium catalyst that produces anhigh molecular weight-high density ethylene polymer film resin that hasgood film properties and good bubble stability at high production ratesand thin gauges has, until this invention, not been accomplished.

It is an object of this invention to provide chromium catalystcompositions.

It is another object of this invention to provide chromium catalystcompositions that are useful for polymerizing ethylene or copolymerizingethylene with a comonomer.

It is yet another object of this invention to provide chromium catalystcompositions that are useful for polymerizing ethylene or copolymerizingethylene with a comonomer, thereby producing a high molecularweight-high density ethylene polymer that is useful for making a filmthat has good film properties and that can be processed at highproduction rates and thin gauges.

In accordance with one embodiment of this invention a chromius catalystcomposition is provided. This chromium catalyst composition comprises atleast two chromium catalyst systems:

(a) One of these chromium catalyst systems comprises chromium and asupport. The amount of chromium in this catalyst system is from about0.5 to about 1 weight percent based on the total weight of said catalystsystem. The support comprises silica and titania. The support has a porevolume from about 2 to about 3 cubic centimeters per gram and a surfacearea from about 400 to about 600 square meters per gram.

(b) Another one of these chromium catalyst systems also compriseschromium and a support. The amount of chromium in this catalyst systemis from about 1 to about 1.5 weight percent based on the total weight ofsaid catalyst system. The support also comprises silica and titania. Thesupport has a pore volume from about 0.7 to about 1.4 cubic centimetersper gram, and a surface area from about 300 to about 400 square metersper gram.

In accordance with another embodiment of this invention the aboveembodiment of the invention can be contacted with ethylene or withethylene and another comonomer, under polymerization conditions, toproduce a homopolymer or a copolymer.

The invention as disclosed in this application can be suitably practicedin the absence of any steps, components, compounds, or ingredients notdisclosed herein.

DETAILED DESCRIPTION OF THE INVENTION

In general, the chromium catalyst compositions used in this inventioncomprise at least two chromium catalyst systems. These chromium catalystsystems comprise a chromium component and a support component. Thesupport component comprises silica and titania. The term "support" isnot meant to be construed as an inert component.

The Chromium Catalyst System that has a High Pore Volume Support

The amount of chromium in this chromium catalyst system is from about0.5 weight percent to about 1 weight percent based on the total weightof the catalyst system (hereafter "Catalyst System XX"). Ifsubstantially less than 0.5 weight percent of chromium is used, thecatalyst system will have an undesirably low activity. If substantiallymore than 1 weight percent is used, the catalyst system will produce aresin that will have an undesirable amount of long chain branching. Aresin with an undesirable amount of long chain branching would be lessdesirable for film applications because of lower film properties such astear resistance in the transverse direction. Additionally, having asubstantially higher amount of chromium in the catalyst system couldimpart an undesired color to the resin.

The support comprises silica and titania. The amount of titania is fromabout 2 weight percent to about 10 weight percent based on the totalweight of the support. If substantially less than 2 weight percent oftitanium is used, the molecular weight distribution of a resulting resinwill be too narrow. If substantially more than 10 weight percent isused, the catalyst system will have an undesirably low activity. Theamount of silica is from about 80 to about 98 weight percent based onthe total weight of the support. Other components can be present in thesupport, provided that, they do not adversely affect the properties ofthe support or adversely affect the properties of the resins produced.

The support should have a pore volume from about 2 cubic centimeters pergram to about 3 centimeters per gram. If the pore volume issubstantially less than 2 cubic centimeters per gram, the melt index ofa resulting resin will be undesirably low. If the pore volume issubstantially more than 3 cubic centimeters per gram the catalyst systemwill be to soft to handle.

The support should have a surface area from about 400 square meters pergram to about 600 square meters per gram. If the surface area issubstantially less than about 400 square meters per gram, the catalystwill have an undesirably low polymerization activity and the amount oflong chain branching in a resulting resin will be undesirably increased.If the surface area is more than 600 square meters per gram, thecatalyst will substantially lose its melt index potential and thecatalyst will have an undesirably low polymerization activity.

These catalyst systems can be produced in accordance with the proceduresdisclosed in U.S. Pat. Nos. 3,887,494; 3,900,457; and 4,119,569; thedisclosures of which are hereby incorporated by reference.

The Chromium Catalyst System that has a Low Pore Volume Support

The amount of chromium in this catalyst system is from about 1 weightpercent to about 1.5 weight percent based on the total weight of thecatalyst system (hereafter "Catalyst YY"). If substantially less than 1weight percent of chromium is used, the amount of long chain branchingdesired in the resulting resin will not be achieved and when the resinis blown into a film, the bubble will be less stable. If substantiallymore than 1.5 weight percent is used, I the resulting resin could havean undesirable color imparted to it.

The support comprises silica and titania. The amount of titania is fromabout 2 weight percent to about 10 weight percent based on the totalweight of the support. If substantially less than 2 weight percent oftitanium is used, the molecular weight distribution of a resulting resinwill be too narrow. If substantially more than 10 weight percent isused, the catalyst system will have an undesirably low activity. Theamount of silica is from about 80 to about 98 weight percent based onthe total weight of the support. Other components can be present in thesupport, provided that, they do not adversely affect the properties ofthe support or adversely affect the properties of the resins produced.

The support should have a pore volume from about 0.7 cubic centimetersper gram to about 1.4 centimeters per gram. If the pore volume issubstantially less than 0.7 cubic centimeters per gram, the catalystsystem will have an undesirably low activity. If the pore volume issubstantially more than 1.4 cubic centimeters per gram the bulk densityof the catalyst will decrease undesirably and the amount of long chainbranching in the resulting resin will decrease undesirably. It ispreferable if the pore volume is from about 0.8 to about 1.2 cubiccentimeters per gram for best control of long chain branching and filmproperties.

The support should have a surface area from about 300 square meters pergram to about 400 square meters per gram. If the surface area issubstantially less than about 300 square meters per gram, the catalystwill have an undesirably low activity. If the surface area is more than400 square meters per gram, the resulting resin will have an undesirablylow film processability.

These catalyst systems can be produced in accordance with the proceduresdisclosed in U.S. Pat. No. 4,981,831 the disclosure of which is herebyincorporated by reference.

It is preferred if the amount of titania in each support issubstantially the same. If the amount of titania is not substantiallythe same, the molecular weight distributions of each of the resultingpolymers will be undesirably different.

The surface area and pore volume of the supports can be determined bynitrogen sorption by a person with ordinary skill in the art. Forexample, the following references can be used "Adsorption, Surface Areaand Porosity" by S. J. Gregg and K. S. W. Sing, Academic Press, London(1982); and "Introduction to Powder Surface Area" by S. Lowell, J. Wiley& Sons, New York, N.Y. (1979); the disclosures of which are herebyincorporated by reference. Additionally, a "Quantachrome Autosorb-6Nitrogen Pore Size Distribution Instrument" can be used to helpdetermine surface areas and pore volumes of the supports. Thisinstrument (or similar instrument) is available from the QuantachromeCorporation, Sysset, N.Y.

Once the chromium catalyst systems are made they can be combined in anymanner known in the art. For example, they can be dry blended togetherin a mixer or added to a feed stream that leads to a reactor. The weightratio of Catalyst XX to Catalyst YY can vary from 99:1 to 1:99 dependingon the properties desired in the resin. In general, using more ofCatalyst XX produces a resin that has better film properties, whereas,using more of Catalyst YY produces a resin that has a better bubblestability during film production. It is preferred if the weight ratio ofCatalyst XX to Catalyst YY is from about 6:1 to about 1:1; morepreferably it is about 5:1 to about 2:1 in order to promote theproduction of resins with good film properties and good filmprocessability.

It should be noted that after the catalyst systems are combined they canbe activated together at the same temperature. They can also beseparately activated and then combined. However, if they are separatelyactivated the temperature at which each activation takes place shouldnot be substantially different, otherwise, the molecular weightdistributions of each of the resulting polymers will be undesirablydifferent.

The chromium catalyst compositions used in this invention can be used tohomopolymerize ethylene or to copolymerize ethylene with anothercomonomer. Suitable comonomers are those olefins having from 3 to about20 carbon atoms. It is preferable if these olefins are 1-olefins.Suitable examples of these olefins include propylene, 1-butane,3-methyl-1-butane, 1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene,1-hexene, 3-ethyl-1-hexene, 1-octane, 1-decene, and mixtures thereof.

Various polymerization schemes are known in the art. For example, U.S.Pat. Nos. 2,825,721; 3,152,872; 3,172,737; 3,203,766; 3,225,023;3,226,205; 3,242,150; 3,248,179; and 4,121,029 (the disclosures of whichare hereby incorporated by reference) disclose several polymerizationschemes. A particularly preferred polymerization scheme is the slurry orparticle form polymerization method. This method is disclosed, forexample, in U.S. Pat. No. 3,248,179.

The substantially monomodal, high molecular weight-high density ethylenepolymer resins produced using the chromium catalyst compositionsdisclosed herein can be made into films that have competitive filmproperties and good processability. These resins will have a high loadmelt index from about 1 to about 20 (preferably about 5 to about 15)grams per ten minutes, when measured in accordance with ASTM D-1238 65T.Additionally, these resins will have a density from about 0.94 to about0.97 (preferably about 0.945 to about 0.955) grams per cubic centimeter,when measured in accordance with ASTM D-1505-85.

EXAMPLE

This example is provided to further assist a person skilled in the artwith understanding this invention. This example is intended to beillustrative of the invention and is not meant to be construed aslimiting the scope of the invention.

Part One

The following chromium catalyst composition was used to copolymerizeethylene and 1-hexene:

(1) A chromium catalyst system that had a high pore volumesilica-titania (2.5 weight percent titania) support was purchased fromthe W. R. Grace Corporation. It had a surface area of about 550 squaremeters per gram and a pore volume of about 2.5 cubic centimeters pergram. It also had a chromium content of about 1 weight percent based onthe weight of the chromium catalyst system; and

(2) A chromium catalyst system that had a low pore volume silica-titania(2.3 weight percent titania) support was purchased from the W. R. GraceCorporation. It had a surface area of about 360 square meters per gramand a pore volume of about 1.1 cubic centimeters per gram. It also had achromium content of about 1.1 weight percent based on the weight of thechromium catalyst system.

These two catalyst systems were then mixed together and activated bysubjecting them to air at 650 degrees Celsius.

The copolymerization was conducted in an 87 liter, 15.2 centimeter pipeloop reactor. The copolymer was recovered in a flash chamber and aVulcan dryer was used to dry the copolymer.

Ethylene that had been dried over alumina was used as the polymerizationmonomer. 1-hexene that had been dried over alumina was used as thecomonomer. Isobutane that had been degassed by fractionation and driedover alumina was used as the diluent. The copolymerization was conductedat about 100 degrees Celsius and lasted for about one hour.

The ethylene-1-hexene copolymer produced was then evaluated as a filmresin. The film blowing conditions were as follows:

1. a die gap of 0.035 inches;

2. a four to one blow up ratio;

3. a fourteen inch frost line height;

4. a screw speed of 75 revolutions per minute; and

5. an extruder temperature of 220 degrees Celsius.

The resin was blown into a one mil film (except when it was down gauged)and compared to other film resins. The results are presented below.

                  TABLE 1                                                         ______________________________________                                        Material        A.sup.1 B.sup.2 C.sup.3                                                                            S1.sup.4                                                                           S2.sup.5                            ______________________________________                                        HLMI (dg/min).sup.6                                                                           7.5     6.4     8.9  8.7  8.8                                 MI (dg/min).sup.8                                                                             0.07    0.04    0.08 --   --                                  HLMI/MI         108     145     111  --   --                                  Density (gms/cc).sup.8                                                                        0.9531  0.9509  0.949                                                                              0.953                                                                              0.951                               Screw RPM       75      75      75   75   75                                  Melt temp. (°C.)                                                                       242     241     234  242  240                                 Rate (lb/hr)    38.9    38.9    40.5 34.2 35.1                                Extruder (kW)   5.1     5.1     4.9  4.5  4.5                                 Die pressure (psi)                                                                            2950    3100    2700 3400 3400                                Line speed (ft/min)                                                                           64      64      67   57   57                                  Gauge (mils)    1       1       1    1    1                                   Maximum line speed* (ft/min)                                                                  --      --      124s 103s 100s                                Gauge @ max. speed (mils)                                                                     --      --      0.5  0.5  0.5                                 Breathing.sup.9 1       1       1    1    1                                   Dancing.sup.10  2       2       1    2    2                                   Streaks.sup.11  2       2       2    1    2                                   Surging.sup.12  1       1       1    1    1                                   Bubble symmetry.sup.13                                                                        1       1       1    1    2                                   Gel uniformity.sup.14                                                                         3       2       1    2    2                                   TOTAL.sup.15    10      9       7    8    10                                  ______________________________________                                         *s indicates a stable bubble at the maximum line speed shown.                 Table Notes:                                                                  .sup.1 This is a film produced from a resin that was made by                  copolymerizing ethylene and 1hexene with only Catalyst XX.                    .sup.2 This is a film produced from a resin that was made by                  copolymerizing ethylene and 1hexene with only Catalyst YY.                    .sup.3 This is the inventive film produced from the inventive resin that      was made from the inventive catalyst composition.                             .sup.4 This is an industry standard film resin.                               .sup.5 This is another industry standard film resin.                          .sup.6 This is the High Load Melt Index measured in accordance with ASTM      D1238 65T.                                                                    .sup.7 This is the Melt Index measured in accordance with ASTM D1238 65T.     .sup.8 This is the Density measured in accordance with ASTM D1505-85.         Items 9-15 are qualitative observations of the bubble during film             production. The range in results is from 1 to 4. A "1" means that the         defect is not detectable or barely visible. A "2" means that the defect i     visible but it is not serious. A "3" means that the defect is not             acceptable but the resin could be processed. A "4" means that the defect      is so bad that it would shut down the line.                                   .sup.9 This is a qualitative measure of the vertical instability in the       frost line position.                                                          .sup.10 This is a qualitative measure of the horizontal instability in th     bubble.                                                                       .sup.11 This is a qualitative measure of the streaks present in the film,     especially along port lines.                                                  .sup.12 This is a qualitative measure of the stability of the resin           through the extruder.                                                         .sup.13 This is a qualitative measure of the symmetry of the bubble in        three dimensions.                                                             .sup.14 This is a qualitative measure of the imperfections in the film        such as, chars, gels, applesauce, or other nonuniform appearance.             .sup.15 This is the total of the qualitative measures.                   

As can been seen from table one, inventive resin C processed extremelywell. The bubble was stable at high line speeds as well. Furthermore,when the film was downgauged to a 0.5 mil thickness the line speed was124 ft/min. with no significant instabilities. Also, the out put ratewas higher than the industry standards by about 5 lbs/hr which is afairly significant difference on in this particular extrusion process.

Part Two:

Several more sample lots of the inventive resin were made according tothe procedure in part one. These sample lots and an industry standardsample were then made into film on the same extrusion line. They wereprocessed under different conditions in order to find the range of filmproperties each sample possessed. The results are presented below intable two.

                  TABLE 2                                                         ______________________________________                                                                 Industry                                                           Inventive Samples                                                                        Standard                                                           Several Lots                                                                             Sample                                               ______________________________________                                        Total Energy Dart Drop                                                        ASTMD-4272      0.8-1.8      1.3-2.9                                          Elemendorf Tear (g)                                                           ASTMD-1922 MD       18-29        17-28                                                   TD        90-250       95-400                                      Tensile at Yield (psi)                                                        ASTMD-882  MD       4450         4450                                                    TD       4250         4100                                         Elastic Mod. (psi)                                                            ASTMD-882  MD        131          136                                                    TD        164          168                                         ______________________________________                                    

As can be seen from the above results the properties of the monomodalinventive resins are competitive with the multicomponent industrystandard resin.

That which is claimed is:
 1. A chromium catalyst composition comprisingat least two chromium catalyst systems wherein:(a) at least one of saidchromium catalyst systems comprises chromium and a support, and whereinthe amount of said chromium is from about 0.5 to about 1 weight percentbased on the total weight of said catalyst system, and wherein saidsupport comprises silica and titania, and wherein said support has apore volume from about 2 to about 3 cubic centimeters per gram, andwherein said support has a surface area from about 400 to about 600square meters per gram; and (b) at least one of said chromium catalystsystems comprises chromium and a support, and wherein the amount of saidchromium is from about 1 to about 1.5 weight percent based on the totalweight of said catalyst system, and wherein said support comprisessilica and titania, and wherein said support has a pore volume fromabout 0.7 to 1.4 cubic centimeters per gram, and wherein said supporthas a surface area from about 300 to about 400 square meters per gram.2. A composition according to claim 1 wherein said supports in (a) and(b) consist essentially of silica and titania.
 3. A compositionaccording to claim 1 wherein said supports in (a) and (b) consist ofsilica and titania.
 4. A composition according to claim 1 wherein saidsupport in (b) has a pore volume from about 0.8 to about 1.2 cubiccentimeters per gram.
 5. A composition according to claim 4 wherein saidsupports in (a) and (b) consist essentially of silica and titania.
 6. Acomposition according to claim 4 wherein said supports in (a) and (b)consist of silica and titania.
 7. A chromium catalyst compositionconsisting essentially of two chromium catalyst systems wherein:(a) oneof said chromium catalyst systems consists essentially of chromium and asupport, and wherein the amount of said chromium is from about 0.5 toabout 1 weight percent based on the total weight of said catalystsystem, and wherein said support consists essentially of silica andtitania, and wherein said support has a pore volume from about 2 toabout 3 cubic centimeters per gram, and wherein said support has asurface area from about 400 to about 600 square meters per gram; and (b)one of said chromium catalyst systems consists essentially of chromiumand a support, and wherein the amount of said chromium is from about 1to about 1.5 weight percent based on the total weight of said catalystsystem, and wherein said support consists essentially of silica andtitania, and wherein said support has a pore volume from about 0.7 to1.4 cubic centimeters per gram, and wherein said support has a surfacearea from about 300 to about 400 square meters per gram.
 8. Acomposition according to claim 7 wherein said supports in (a) and (b)consist of silica and titania.
 9. A composition according to claim 7wherein the support in (b) has a pore volume from about 0.8 to about 1.2cubic centimeters per gram.
 10. A composition according to claim 9wherein said supports in (a) and (b) consist of silica and titania.